The use of torque transfer devices in association with the operation of a vehicular transmission is well known in the art. The transmission of a vehicle functions to deliver the power received from an engine through a progressive variety of gear ratios so as most effectively to drive one or more axles of the vehicle. In a vehicle with an automatic transmission (a transmission system in which the driver does not directly control when the gear ratios are to be changed), a control unit senses the speed of the vehicle and determines when the torque transfer devices--typically clutches or brakes--that control the speed changing gears are to be activated or deactivated. To activate or deactivate these torque transfer devices by fluid pressure, various types of solenoid valves may be used.
Prior art control systems in hydraulically actuating transmissions typically utilize pulse width modulating (PWM) solenoid valves actuated from the transmission controller directly to control oil pressure applied to torque transfer devices.
It is well known that a solenoid is a length of wire that is coiled such that when an electrical current flows through the wire a movable core is drawn into the coil. The solenoid is, therefore, readily capable of opening or closing either a normally open or a normally closed valve, and the solenoid operating variety of such valves are commonly designated as solenoid valves.
Torque transfer devices--whether operating as clutches or brakes--used in an automatic transmission utilize a force, such as that supplied by pressurized fluid, to activate and deactivate the torque transfer device. Torque transfer devices have two relatively movable parts that may be engaged or disengaged without bringing either of the moving parts to rest. Typically, the pressurized oil is applied to force one of the moving parts into a frictionally driving relationship with the other moving part, and at the desired speed. To achieve this, the transmission controller activates a desired solenoid valve in the transmission system by applying a voltage differential across the coil such that the solenoid valve is activated from either its normally closed or normally open state.
In normal operation, the transmission controller modulates the PWM solenoid voltage at a constant frequency with a varying duty cycle (also known as voltage signal amplitude) as it delivers the fluid pressure to the torque transfer device. Unfortunately, when the voltage signal--and thus the fluid pressure--is modulated at a constant frequency, a corresponding torque spike may be transmitted throughout the entire drive system of the vehicle. Consequently, these torque spikes will generally result in a "bad shift" (the moving parts of the torque transfer device do not properly align) or a "loud shift" (the moving parts of the torque transfer device make noise as they become properly aligned). This drawback is emphasized if the torque spikes are of the correct frequency or magnitude to "excite" the vehicle system. Rotating torque transfer devices are particularly susceptible to bad shift and loud shift problems, which combine to cause shift "growl". Shift "growl" is most prevalent in transmissions used in light stiff vehicles such as a bus.
To eliminate these torque spike problems, the prior art teaches the use of spring packs to absorb the bad or loud shifting caused by modulating the fluid pressure at a constant frequency. Alternatively, more expensive proportional solenoid valves can be incorporated into the transmission system to smooth out the resulting torque spikes. Other hardware, such as A/R valves, accumulator springs and hydraulic accumulators, may be utilized to provide a smoother quieter ride for the vehicle system.
While attempts have been made to provide smooth running, noise-free automatic transmissions, the prior art has not provided a controller which assures a smooth running transmission. The present invention provides a quiet smooth operation of the transmission by employing pulse width modulating solenoid valves controlled so as to prevent such unfavorable shifting characteristics.